[ELF] - Linkerscript - fix handling of OUTPUT_ARCH command.

OUTPUT_ARCH command can contain architecture values separated with ":", like:
OUTPUT_ARCH(i386:x86-64)

We did not support that, because got 3 lexer tokens here after recent changes.

This trivial patch fixes the issue, now whole expression inside 
OUTPUT_ARCH is just ignored.

Differential revision: https://reviews.llvm.org/D29640

llvm-svn: 294432